I. Field of the Invention
The present invention relates to imaging devices. In particular, the present invention relates to devices that detect and provide a measure of an intensity of light impinging on imaging pixel devices.
II. Background Information
Imaging arrays, used to produce an image representing an object, are typically formed of rows and columns (bitlines) of photo detectors (pixels). The pixels generate photo charges proportional to light reflected from an object to be imaged. Photo charges from each pixel are converted to a signal (charge signal) or potential representative of a level of energy reflected from a respective portion of the object. The signal or potential is read and processed by video processing circuitry to create an image representing an object.
Pixels belongings to a same bitline are usually connected at a common output node from where a signal or potential, representative of the level of energy, is read out. Pixels belonging to the same bitline xe2x80x9cseexe2x80x9d an overall capacitance (hereinafter referred to as xe2x80x9cbitline capacitancexe2x80x9d), at the common output node. Each pixel in a same bitline is individually controlled to read out at the common output node. Typically, pixels belonging to a same row are commonly controlled by a same signal (wordline) such that an entire row may be read out at a substantially same time.
To meet the increasing need for high speed image sensor devices, image sensor arrays are integrated with digital circuitry that controls the operation of the array and processes the array""s output. Integration of image sensors with complementary-metal-oxide-semiconductor (CMOS) support circuitry is most desirable because of the low power consumption characteristics and common availability of CMOS technology. Such an imaging array integrated with CMOS support circuitry is called CMOS active pixel sensor (APS) array.
Typically, a pixel includes a photosensor that detects light impinging thereon and xe2x80x9cconvertsxe2x80x9d the light into an electronic signal indicative of an intensity of light detected by the pixel. A driving device receives the electronic signal and drives a current proportional to the electronic signal to a bitline to which the pixel is coupled. Then the pixels in a selected row are accessed by asserting the WORDLINE signal to each pixel access device of each pixel cell of a selected row. Then each bitline to which a corresponding pixel of the selected row is coupled, may be charged by a current driven by the driving device of the pixel to a voltage level representative of an intensity of light detected by that pixel. The pixels of an entire row may thus be read out at a substantially same time. The pixel cells of other rows, not currently accessed, have their pixel access devices switched off by deasserting the wordline signals corresponding to these rows.
One problem with conventional read out circuits is that the read out signal at the output of the read out circuit is not linear with the input signal which reflects the intensity of the light impinging on a pixel. The non-linearity of such read out circuits negatively affects the accuracy of the image read out by these circuits. The lack of linearity is mainly due to the body effect that affects conventional read out circuits that use source follower schemes. The body effect is a well-known principle in Metal Oxide Semiconductor Field Effect Transistors (MOSFET)xe2x80x94the threshold voltage of a MOSFET varies in accordance with variations in the source-to-bulk voltage of such transistor. In a source follower circuit, such as the one implemented in conventional read out circuits, this effect causes non-linearity in the output voltage Vout, as Vout=Vinxe2x88x92Vth, and the threshold voltage Vth varies with the output voltage Vout. Furthermore, read out circuits that use source follower circuits generate at their output voltages Vout that are lower in value than the input voltages Vin due to the threshold voltage of the transistors employed by these circuits.
Briefly, one embodiment of the present invention includes a read out circuit. The read out circuit includes an amplifier that has a first and second input devices and an output port. The first input device is included in a pixel of an active pixel sensor array to receive an input signal indicative of an intensity of light detected by the pixel. The amplifier generates at an output thereof an output signal proportional to the input signal.